This invention relates to sound masking systems and, in particular, to sound masking systems for open plan offices.
Freedom from distraction is an important consideration for workers' satisfaction with their office environment. In a conventional enclosed office with full height partitions and doors, any speech sound intruding from outside the office is attenuated or inhibited by the noise reduction (NR) qualities of the wall and ceiling construction. Background noise, such as from the building heating or ventilating (HVAC) system, typically masks or covers up residual speech sound actually entering the office. Under normal circumstances, even very low levels of background nose reduce audibility of the residual speech to a sufficiently low level that the office worker is unable to understand more than an occasional word or sentence from outside and is, therefore, not distracted by the presence of colleagues' speech. In fact, it was shown more than 35 years ago that a standardized objective measure of speech intelligibility called the Articulation Index, or AI, reliably predicts most peoples' satisfaction with their freedom from distraction in the office. “Perfect” intelligibility corresponds to an AI of 1.0, while “perfect” privacy corresponds to an AI of 0.0. Generally, office workers are satisfied with their privacy conditions if the AI of intruding speech is 0.20 or less, a range referred to as “normal privacy” or better.
In recent years, the “open plan” type of office design has become increasingly popular. The open plan design includes partial height partitions and open doorways between adjacent workstations. Due to its obvious flexibility in layout and its advantages in enhancing communication between co-workers, the open plan office design is increasingly popular. However, despite the advantages of the open plan type office, unwanted speech from a talker in a nearby workstation is readily transmitted to unintended listeners in nearby workstation areas.
To reduce the level of unwanted speech in open plan offices, some limited acoustical measures can be employed. For example, highly sound absorptive ceilings reflect less speech, higher partitions attenuate direct path sound signals, particularly for seated workers, and higher partitions also diffract less sound energy over their tops. Additionally, the open doorways can be placed so that no direct path exists for sound transmission directly from workstation to workstation, and the interiors of workstations can be treated with sound absorptive panels. Nevertheless, even in an acoustically well designed open office, the sound level of intruding speech is substantially greater than in an enclosed office space. One other important method that can be used to obtain the normal privacy goal of 0.20 AI in an open plan office is to raise the level of background sound, usually by an electronic sound masking system.
Conventional sound masking systems typically comprise four main components: an electronic random noise generator, an equalizer or spectrum shaper, a power amplifier, and a network of loudspeakers distributed above the office, usually in the ceiling plenum. The equalizer adjusts the white noise spectrum provided by the electronic random noise generator to compensate for the frequency dependent acoustical filtering characteristics of the ceiling and plenum and to obtain the sound masking spectrum shape desired by the designer. The power amplifier raises the signal voltage to permit distribution to the loudspeakers without unacceptable loss in the network lines and ceiling tiles. The generator, equalizer, and power amplifier may be integrated with a speaker or may be located at a central location connected to the loudspeaker distribution network.
The goal of any sound masking system is to mask the intruding speech with a bland, characterless but continuous type of sound that does not call attention to itself. The ideal masking sound fades into the background, transmitting no obvious information. The quality of the masking sound of all currently sold devices is subjectively similar to that of natural random air turbulence noise generated by air movement in a well-designed heating and ventilating system. By contrast, if it has any readily identifiable or unnatural characteristics such as “rumble,” “hiss,” or tones, or if it exhibits obvious temporal variations of any type, it readily becomes a source of annoyance itself.
Obtaining the correct level or volume of the masking sound also is critical. The volume of sound needed may be relatively low intensity if the intervening office construction, such as airtight full height walls, provides a high NR. However, the volume of the masking sound must be a relatively high intensity if the construction NR is reduced by partial-height intervening partitions, an acoustically poor design or layout, or materials that have a high acoustic reflectivity. Even in an acoustically well designed open office, the level of masking noise necessary to meet privacy goals may be judged uncomfortable by some individuals, especially those with certain hearing impairments. However, if the masking sound has a sufficiently neutral, unobtrusive spectrum of the right shape, the intensity of the masking sound can be raised to a sound level or volume nearly equal to that of the intruding speech itself, effectively masking it, without becoming objectionable.
Subjective spatial quality is another important attribute of sound masking systems. The masking sound, like most other natural sources of random noise, must be subjectively diffuse in quality in order to be judged unobtrusive. Naturally generated air noise from an HVAC system typically is radiated by many spatially separated turbulent eddies generated at the system terminal devices or diffusers. This spatial distribution of sources imparts a desirable diffuse and natural quality to the sound. In contrast, even if a masking system provides an ideal spectrum shape and sound level, its quality will be unpleasantly “canned” or colored subjectively if it is radiated from a single loudspeaker or location. A multiplicity of spatially separated loudspeakers radiating the sound in a reverberant (sound reflective) plenum normally is typically used in order to provide this diffuse quality of sound. Almost all plenums use non-reflective ceiling materials and fireproofing materials and require two or more channels radiating different (incoherent) sound from adjacent loudspeakers in order to obtain the required degree of diffusivity. Each loudspeaker normally serves a masking zone of about 100–200 square feet each (i.e. placed on 10′ to 14′ centers). In most cases, the plenum space above the ceiling is an air-return plenum so that the loudspeaker network cable must be enclosed in metal conduit or use special plenum-rated cable in order to meet fire code requirements.
A typical system diffuses the acoustic sound masking signal by placing the loudspeakers in the plenum space facing upward to reflect the acoustic masking signal off the hard deck. As a result, direct path energy from the location of a loudspeaker to the ear of the listener is intentionally minimized by the acoustic sound masking signal that propagates substantially throughout the above ceiling volume and filters down through the ceiling and ceiling elements such as light fixtures, mechanical system grilles, return air openings, etc., at locations somewhat removed from the loudspeaker location. The effectiveness of this approach to diffusion depends on several characteristics. These include the directivity characteristics of the loudspeakers, elements in the plenum such as mechanical system ducts, and on the physical characteristics of the ceiling material itself, such as its density and upper surface acoustical absorption. Costly measures are sometimes needed to improve the uniformity and diffuseness of the masking sound. Some of these measures include employing special vertically directional baffles for the loudspeakers to spread the sound horizontally and coating the upper surface of the ceiling tile with special foils to further spread out the masking sound horizontally. In high density ceilings with large openings for HVAC return air, specially designed acoustical grill “boots” are often necessary to avoid excessive concentration of masking sound, or “hot spots.”
In addition, the sound attenuation characteristics of the ceiling assembly are normally not knowable until after installation and testing. Since masking system loudspeakers are normally installed before the ceiling for reasons of access and economy costly adjustable frequency equalization for the masking sound must be provided to compensate for these site-specific characteristics. Thus, additional time and cost are incurred due to the testing and frequency adjustment that must be performed post installation.
Also, because the acoustic sound masking signal must pass through the acoustical ceiling and be attenuated thereby, a large part of the acoustical power radiated by the loudspeakers is wasted in the form of heat as the acoustic masking signal is attenuated. Accordingly, despite the requirement for only very small amounts of acoustical sound masking power within the listening space itself, relatively high power electrical signals driving large and costly loudspeakers are needed to provide the necessary masking signal strength. Due to the power required, the loudspeaker assemblies are normally large and heavy. Thus, in addition to the costs incurred by the larger amount of power required, the loudspeaker and its enclosure must be supported from additional structure rather than directly by the ceiling tile in order to avoid sagging of the lightweight ceiling material. This additional support structure increases the installation cost, and the placement of the large loudspeakers in the plenum area inhibits access to the above ceiling space, which also complicates the design and installation of the loudspeakers.
Masking loudspeakers sometimes have been installed below higher ceilings, or within the ceiling, in order to overcome some of these limitations. However, their use has been restricted to installation in facilities with atypically high ceiling heights due to appearance, masking sound uniformity, an overly small or crowded plenum area, and cost considerations. When a conventional loudspeaker is attempted below a ceiling in a more typical office environment with ceiling heights of 9′–12′, or within the ceiling, the uniformity of masking sound is found to be unacceptable. In particular, conventional loudspeakers exhibit a narrow beamwidth at higher frequencies, causing “hot-spotting” on their axes. Unlike music or other time varying signals, masking sound has essentially constant bandwidth temporally, and any significant narrowing of beamwidth within the acoustic band is immediately obvious and unpleasant to most individuals. Moreover, unless loudspeakers are mounted within several feet of one another, overall level uniformity is unacceptable due to square law or distance spreading, that is, the sound level attenuates unacceptably with distance from the loudspeaker, drawing attention to its location. This close loudspeaker proximity is unsightly and uneconomic. Thus, in these systems an unacceptable number of these conventional loudspeakers are required to avoid hot-spotting and signal non-uniformity within a masking zone.
Sound masking spectra normally used in open plan offices are well documented. For example, see L. L. Beranek, “Sound and Vibration Control”, McGraw-Hill, 1971, page 593. These spectra were empirically derived over a period of a number of years and are characterized by relatively high levels of sound at lower speech frequencies and by relatively low levels of sound at the higher speech frequencies. Such spectra have been found to provide both effective masking of speech sound intruding into an office and unobtrusive quality of masking sound when used in a typical office with sufficiently high partial height office partitions that act as acoustical barriers between work stations, particularly at high frequencies. These spectra have also been found to work adequately in some other office settings with sufficient high frequency inter-office speech attenuation.
The masking sound level considered unobtrusive by most open office occupants is approximately 48 dBA sound pressure level. As masking levels are increased above 48 dBA, complaints of excessive masking sound increase. Unfortunately, it can be shown that this level of sound with the typically used spectrum is largely ineffective for sound masking in an office setting without significant acoustical barriers to reduce high frequencies of intruding speech sound. If barriers are low or absent, the required distance between workstations to obtain normal speech privacy conditions may exceed 20 feet or more, even with a high quality sound masking system using a typical sound masking spectrum.
Therefore, it would be advantageous to provide a sound masking system that is easier to install, requires fewer adjustments, requires fewer components than the conventional sound masking systems, and provides more privacy in an open plan office.